The OS which powers my E51 and E61i phone is now open sourced.
From the Symbian.org wiki site, the whole system platform is ready for download and development.
Amounting to about 25 million lines of code (38 million lines if you include the tools used to create the platform), Symbian now becomes one of the largest projects ever to be moved from proprietary to open source.
The not so good news is that you cannot reprogram your existing phone with Symbian 3 (this is symbian with all the proprietary 3rd party IP removed). However, you can take pieces of the individual applications, modify and extend them and install them back in the phone.
Greater news is that it is possible to build a Symbian phone now. The Symbian foundation provides template base ports for certain types of hardware. This includes an emulator run in QEMU.
All in all, this is great news for an OS installed in a majority of phones worldwide.